This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Introduction

Ewing's sarcoma of the calcaneus is rare. About thirty cases with calcaneus involvement
have been reported in the literature. Talus skip metastases have rarely been described
in the available literature

Case presentation

We report a case of a 14-year-old Moroccan boy, who presented with Ewing's sarcoma
of his right calcaneus, diagnosed by swelling of the calcaneus evolving over a year.
Radiography, computed tomography and magnetic resonance imaging showed an important
tumoral process of the calcaneus and talus skip metastases. The diagnosis was confirmed
with histology after a biopsy. In spite of amputation and postoperative chemotherapy,
our patient died six months later due to secondary respiratory distress after lung
metastasis.

Conclusion

Imaging, especially magnetic resonance, is important in the diagnosis of Ewing sarcoma
and skeletal skip metastases. Treatment of Ewing's sarcoma consists of chemotherapy,
radiation therapy and surgical resection depending on the stage and extent of the
disease. With the exception of lesions in the calcaneus, the prognosis for disease-free
survival of Ewing's sarcoma of the foot is excellent.

Introduction

Ewing's sarcoma is a rare malignant bone tumor that may affect any bone, usually occurring
in long bones, pelvis and ribs, with only 3-5% of cases in the bones of the hands
and feet [1]. It is a highly anaplastic round-cell tumor, primarily arising in the intramedullary
portion of the bone.

Case presentation

A 14-year-old Moroccan boy presented with painful swelling of his right foot of 12
months duration. A general examination was unremarkable, while local examination revealed
a diffuse swelling involving his right ankle joint and foot. The overlying skin was
normal. The swelling was tender and mobility at the joint was restricted. Hematological
and biochemical investigations revealed a normal hemogram and normal liver and renal
function tests.

Anteroposterior (Figure 1) and lateral radiographs (Figure 2) showed a condensed lesion in the calcaneus of his right foot with aggressive periosteal
reaction and soft-tissue swelling. Computed tomography (CT) revealed a soft-tissue
mass of the foot originating from his calcaneus and a sclerotic lesion of the entire
bone with aggressive spiculated periosteal reaction and cortical destruction (Figure
3). A large soft-tissue mass around the involved bone was indicative of Ewing's sarcoma.
Magnetic resonance imaging (MRI) was then performed and showed a hypointense tumor
mass on T1-weighted sequences (Figure 4) and hyperintense properties on T2-weighted spin-echo sequences compared to surrounding
musculature (Figure 5), a signal pattern characteristic of most tumors. The skip lesions of the talus displayed
hyposignal properties on T1- and T2-weighted sequences. After intravenous gadolinium
chelate administration, strong contrast enhancement of the tumor was observed (Figure
6). Skip metastases of the talus were evidenced as low-signal masses with peripheral
enhancement (Figure 6).

Figure 1.Anteroposterior radiographs of patient's foot show a lesion in the calcaneus condensed
with aggressive periosteal reaction and soft-tissue swelling.

Figure 2.Lateral radiographs of the patient's foot show a condensed lesion in the calcaneus
with aggressive periosteal reaction and soft-tissue swelling.

Figure 3.CT image of the patient's foot, revealing a soft-tissue mass originating from the
calcaneus, permeative destruction of the entire bone with aggressive spiculated periosteal
reaction and cortical destruction.

Figure 4.MRI of the patient's foot shows a hypointense tumor mass on T1-weighted spin-echo
sequences compared to surrounding musculature. The skip lesion of the talus displays a hyposignal on T1-weighted sequences.

Figure 5.MRI of the patient's foot shows a hyperintense tumor mass on T2-weighted spin-echo
sequence images compared to surrounding musculature. The skip lesion of the talus displays a hyposignal on T2-weighted sequences.

Figure 6.T1-weighted fat saturation sequence after intravenous gadolinium chelate administration
reveals strong contrast enhancement of the tumor. We note the skip lesion in the talus as a low signal mass with peripheral enhancement.

A biopsy was performed and histopathology showed a malignant small round-cell tumor,
identified as Ewing's sarcoma at immunohistochemistry study. Chest radiography and
liver ultrasonography excluded the presence of any distant metastases. Our patient
started neoadjuvant chemotherapy and underwent a below-knee amputation. Postoperative
histology confirmed the diagnosis. Our patient remained disease-free for six months
after diagnosis. Based on these findings, a diagnosis of Ewing's sarcoma of the calcaneus
was made.

Discussion

Ewing's sarcoma is a rare malignant neoplasm, predominantly affecting young patients
of the ages five to 20 years. It involves the diaphyses of long bones and occurs less
commonly in flat bones [1]. Clinical and laboratory features include local pain, soft-tissue swelling and erythema,
occasionally accompanied with fever, anemia, leukocytosis, and accelerated erythrocyte
sedimentation rate [2]. It rarely affects the feet.

Cook listed 29 cases of Ewing's sarcoma of the calcaneus in the literature since 1921
[3]. These rare cases are usually misdiagnosed, leading to treatment delay, which is
detrimental to the outcome.

According to a retrospective study concerning 235 patients with non-metastatic Ewing's
sarcoma of the bone, 15 patients were identified with a skip lesion at diagnosis.
However, the skip lesions were located in adjacent juxta-articular bone in only two
cases [4].

The radiographic features of Ewing's sarcoma in our case were those of classic Ewing's
sarcoma: a permeative, lytic and condensed lesion with cortical destruction, aggressive
periosteal reaction, large extraosseous soft-tissue component and often sclerotic
reaction [5,6]. In spite of clinical and radiological findings, Ewing's sarcoma can be misinterpreted
as osteomyelitis, cartilaginous tumor, giant cell lesion, lymphoma or osteosarcoma,
and the distinction often requires extensive evaluation using varied imaging modalities
[7].

CT can reveal a soft-tissue mass of the foot, such as permeative lytic lesions of
the bone with aggressive periosteal reaction and cortical destruction, but the distinction
between osseous remnants, reactive changes and tumor matrix can sometimes be challenging
[8]. Bone scintigraphy of the whole skeleton demonstrates a focus of increased uptake
of technetium-99 m-methylene diphosphonate [8,9].

T2-weighted MRI cannot adequately distinguish tumor from necrosis, and lesion boundaries
are frequently overestimated because of the presence of edema and hemorrhage [9]. The enhancement pattern after administration of contrast medium on MRI allows differentiation
between a tumor and peritumoral reactive edema. Furthermore, MRI can often distinguish
the large solid sarcomatous soft-tissue mass around the involved bone from edema or
an osteomyelitic abscess. MRI findings can narrow the differential diagnosis, but
a specific diagnosis can rarely be established. Therefore, a biopsy of the tumor with
histopathological analysis is needed to confirm the diagnosis. Staging, prior to biopsy,
is essential to document the local and distant spread of the tumor. In Ewing's sarcoma,
the metastatic pattern may be pulmonary involvement, bone or bone marrow spreading,
skip metastases, or combined metastatic disease [7,9].

The imaging features of local spread of Ewing's sarcoma, involving small bones to
adjacent bones, have not been described in the recent literature. It wasn't possible
to determine the exact local extent of the tumor by means of conventional radiography
and CT [5,9].

Due to its superior contrast resolution and multiplanar capabilities, MRI is more
sensitive than other imaging techniques, especially for the investigation of tumor
spread to bony structures and bone marrow. MRI should always be performed in the analysis
of Ewing's sarcoma since it allows accurate evaluation of the tumor extent, which
is decisive for treatment [10].

Skip lesions in patients with otherwise non-metastatic skeletal Ewing's sarcoma may
be of the same importance as the molecular detection of marrow metastases, and possibly
confer a worse prognosis. Newer imaging modalities like positron emission tomography-computed
tomography and careful staging work-up may indicate that skip metastases in Ewing's
sarcoma are more common than previously suspected [8,10].

Conclusions

This case report confirms that the routine radiological management of Ewing's sarcoma
should include radiography and MRI of the affected region, together with whole skeleton
bone scintigraphy and CT of the chest. MRI is essential in the determination of the
true extent of the tumor. It is important to bear in mind that early recognition of
an unusual appearance and location of Ewing's sarcoma is necessary for its adequate
treatment.

Abbreviations

CT: computed tomography; MRI: magnetic resonance imaging

Consent

Written informed consent was obtained from the father of our patient for publication
of this case report and any accompanying images. A copy of the written consent is
available for review by the Editor-in-Chief of this journal.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

HJ, ZB and HE made, analyzed and interpreted our patient's imaging examinations. MM
and TF are the traumatologists whom operated on our patient and made major contributions
to the manuscript. The manuscript was prepared by HJ under the supervision of OE and
AO. All authors read and approved the final manuscript.

Acknowledgements

We wish to acknowledge Prof. Arak Abdelfattah for his critical appraisal.